This invention relates to a plate heat exchanger, which comprises a plurality of plates arranged to the side of each other and each consisting of two metal sheets, which are welded one to the other along two parallel edges and have longitudinal bulgings in parallel with said edges. The present invention has the object to produce a plate heat exchanger where the medium flowing within the plate is guided in a direction in parallel with the extension of the bulgings and back in the same direction. This arrangement implies the further gain that substantially all surfaces of the plates flowed through by a medium participate in the heat exchange. This is not the case, for example, when at one end of the plate a box is used for turning the medium flowing through the plates.